Mesh-adjustable molecular sieve

ABSTRACT

A metal-organic framework-based mesh-adjustable molecular sieve (MAMS) exhibiting a temperature-dependent mesh size. The MAMS comprises a plurality of metal clusters bound with a plurality of amphiphilic ligands, each ligand comprising a hydrophobic moiety and a functionalized hydrophilic moiety, and wherein the metal clusters and amphiphilic ligand functionalized hydrophilic moieties form a metal cluster layer, the metal cluster layer forming at least one hydrophilic pore. On each side of the metal cluster layer, a plurality of associated amphiphilic ligand hydrophobic moieties cooperate with the metal cluster layer to form a tri-layer and a plurality of tri-layers are packed in a facing-spaced apart relationship to form at least one hydrophobic pore.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a divisional application of, and claims priority to, U.S. patent application Ser. No. 11/738,730, filed Apr. 23, 2007, now U.S. Pat. No. 7,687,432, which is a non-provisional application of, and claimed priority to, U.S. Patent Application No. 60/899,115, the disclosures of which are incorporated herein by reference as if fully rewritten herein.

STATEMENT REGARDING FEDERALLY-SPONSORED RESEARCH AND DEVELOPMENT

This invention was made in the course of research partially supported by a grant from the National Science Foundation (CHE-0449634). The government has certain rights in this invention.

REFERENCE TO AN APPENDIX

(Not Applicable)

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates generally to metal-organic framework-based molecular sieves for selective gas adsorption and particularly to such sieves which provide dynamic mesh sizes which are continuously adjustable.

2. Description of the Related Art

Gas separation is an important operation in many industries and conventional processes include distillation, absorption, and molecular sieves. However, using such processes to separate, for example, mixtures comprising chemical pairs of similarly-sized molecules or those with similar boiling points such as ethylene and propylene, methane and ethylene, nitrogen and methane, nitrogen and oxygen, hydrogen and carbon monoxide, and hydrogen and nitrogen can be difficult.

Recently, metal-organic frameworks (MOFs) have been utilized to produce porous materials suitable as molecular sieves for adsorbing specific molecular species. Such frameworks, which comprise metal clusters linked together in a reticular structure with linking ligands, can provide predetermined pore size and functionality. However, when the size disparity of the gas pairs to be separated is small, a molecular sieve with the optimum mesh size is not always readily available and a mismatch inevitably leads to an inefficient operation. Furthermore, the mesh size of even MOFs is fixed upon activation. Therefore, the need exists for a mesh-adjustable molecular sieve (MAMS) capable of dynamically providing a continuum of mesh sizes. Particularly, the need exists for a MAMS having a continuum of mesh sizes in the range of most commercially-important gas separations.

BRIEF SUMMARY OF THE INVENTION

The invention is a metal-organic framework-based (MOF) mesh-adjustable molecular sieve (MAMS). MAMSs exhibit permanent porosity, characterized by strong bonds, and exhibiting temperature-induced dynamically-flexible molecular gating effects, requiring weak interactions, to enable selective adsorption of molecules of interest. A MAMS comprises a layered, generally graphitic, structure wherein the atoms of each layer are connected coordinatively but the layers are held together with weak interactions. Such a structure is obtained by, first, employing an amphiphilic ligand comprising a hydrophobic end and a hydrophilic end, but with the hydrophilic end functionalized with, for example, groups such as carboxylates. The ligand acts as one of two secondary building units (SBUs). The functional groups of the ligand bind metal ions/clusters (the second of the two SBUs) and the structure propagates into a generally two-dimensional sheet or layer. Two layers of ligands sandwich a metal ion/cluster layer to define three-dimensional tri-layers which pack through van der Waals interaction. The tri-layer, then, comprises a hydrophilic cluster layer sandwiched by two hydrophobic ligand layers. Essentially linear hydrophilic pores are created in the middle layer, and the packed tri-layers, with exposed hydrophobic exteriors, define hydrophobic pores. The hydrophobic pores are accessible through the hydrophobic/hydrophilic interface where the hydrophobic ends of adjacent ligands generally point toward each other. The MAMS exhibits a temperature-dependent mesh size which is substantially continuously and dynamically variable in a region of interest of between about 2.9 Å and about 5 Å.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

FIG. 1 a is a conceptual three-dimensional graphical rendition of an exemplary MAMS illustrating the molecular structure and the defined pores.

FIG. 1 b is a simplified conceptual three-dimensional graphical rendition of an exemplary MAMS.

FIGS. 2 a-2 c are conceptual two-dimensional graphical renditions of an exemplary MAMS illustrating the defined pores, activation, and separable molecules.

FIG. 3 a is a three-dimensional graphical rendition of an exemplary octa-nickel metal cluster prior to activation.

FIG. 3 b is a three-dimensional graphical rendition of an exemplary octa-nickel metal cluster after activation.

FIG. 3 c is a three-dimensional graphical rendition of an exemplary metal cluster layer before and after activation.

FIGS. 4 a and 4 b are conceptual three-dimensional graphical renditions of two views of a possible exemplary molecular gate-like structure.

FIG. 5 is a three-dimensional graphical rendition of an exemplary bi-metal SBU.

FIG. 6 is a ChemDraw® (Cambridge, Mass.) chemical structure drawing (ACS (American Chemical Society)-style) rendition of the synthesis of 4′-methyl-biphenyl-3,5-dicarboxylic acid (H₂MBPDC).

FIG. 7 is a ChemDraw® (Cambridge, Mass.) chemical structure drawing (ACS-style) rendition of the synthesis of 4′-tert-butyl-biphenyl-3,5-dicarboxylic acid (H₂BBPDC).

FIGS. 8 a-8 e are ChemDraw® (Cambridge, Mass.) chemical structure drawing (ACS-style) renditions of a number of exemplary ligands.

FIGS. 9 a-9 f illustrate gas adsorption isotherms for selected molecular species mixtures at selected temperatures for Ni₈(μ₃-OH)₄(5-tert-butyl-1,3-benzenedicarboxylate)₆ (MAMS-1).

FIG. 10 illustrates a thermogravimetric analysis (TGA) of MAMS-1.

FIG. 11 illustrates X-ray powder diffraction patterns for fresh MAMS-1 and MAMS-1 activated at 200 deg. C.

FIG. 12 illustrates H₂ adsorption isotherms at 77 deg. K for fresh MAMS-1, MAMS-1 activated at 120 deg. C., and MAMS-1 activated at 200 deg. C.

FIG. 13 illustrates CO₂ adsorption isotherms at 195 deg. K for fresh MAMS-1 and MAMS-1 activated at 200 deg. C.

FIG. 14 illustrates a N₂ adsorption isotherm at 77 deg. K for MAMS-1 activated at 120 deg. C.

FIG. 15 illustrates a C₂H₄ adsorption isotherm at 175 deg. K for MAMS-1.

FIG. 16 illustrates iso-C₄H₁₀ and SF₆ adsorption isotherms at 295 deg. K for MAMS-1.

FIG. 17 illustrates H₂ sorption isotherms (adsorption and desorption) at 77 deg. K for MAMS-1.

FIG. 18 illustrates CH₄ sorption isotherms (adsorption and desorption) at 143 deg. K for MAMS-1.

FIG. 19 illustrates C₂H₄ sorption isotherms (adsorption and desorption) at 195 deg. K for MAMS-1.

FIG. 20 illustrates the temperature-dependent gate opening effect of MAMS-1.

FIG. 21 illustrates cell various cell parameters at different temperatures for MAMS-1.

DETAILED DESCRIPTION OF THE INVENTION

As shown in conceptual exemplary fashion in FIGS. 1 a, 1 b, and 2 a-2 c, the present invention relates to metal-organic framework-based (MOF) mesh-adjustable molecular sieves (MAMS) (e.g., 10, FIGS. 1 a and 1 b and FIGS. 2 a-2 c) suitable for selective gas adsorption and comprising metal clusters or metal ions (e.g., 12, FIGS. 1 a and 3 a-3 c-5) cooperating with amphiphilic ligands (e.g., FIGS. 8 a-8 e), which ligands comprise a hydrophobic moiety (e.g., 14, FIGS. 1 a, 4 a, and 4 b) and a functionalized hydrophilic moiety (e.g., FIGS. 8 a-8 e) wherein the metal clusters or metal ions 12 are coordinatively bound with functionalized hydrophilic moieties to form metal cluster layers 20, which each metal cluster layer 20 forms at least one hydrophilic pore 16, on each side of the metal cluster layer 20, associated amphiphilic ligand hydrophobic moieties 14 cooperate with the metal cluster layer 20 to form a tri-layer 22, and the tri-layers 22 are packed in a facing, spaced-apart relationship to form at least one hydrophobic pore 18 capable of adsorbing gas molecules of interest (e.g., FIGS. 2 a-2 c). In general, based upon the results and observations presented and discussed herein, while not wishing to be bound by any particular theory, it is believed a temperature-dependent gating effect arises from the positioning of the hydrophobic moieties 14 of the amphiphilic ligands. As shown in exemplary FIGS. 2 a-2 c, when the gates 26 are open the hydrophilic pores 16 and hydrophobic pores 18 are all connected, giving rise to a three-dimensional container with space continuity, which would account for the high uptake of selected molecular species (e.g., FIG. 9 a). (See, also FIGS. 1 a, 4 a, and 4 b.) It is likely that the molecular sieving effect comes, at least partially, from the hydrophobic moieties 14 of the amphiphilic ligands. (FIGS. 8 a-8 e.) For example, in view of the kinetic diameters of H₂ (2.89 Å), O₂ (3.46 Å), N₂ (3.64 Å), CO (3.76 Å), one may infer the gate opening 26, 27 for exemplary MAMS-1 (described in detail below) is about 3.0 Å to about 3.4 Å. (FIG. 9 a.)

The metal is selected from the group consisting of transition metals, Al, Ge, and Ga and the amphiphilic ligand (e.g., FIGS. 8 a-8 e) is selected from the group consisting of:

wherein X is at least one of carboxylate, cyano, and phosphate, sulfonate, imidazolate, pyridine, pyrazole, and tetrazolate and Y is at least one of tert-butyl, methyl, isopropyl, trifluoromethyl, butoxyl, and butylsulfonyl, alkyl, halogenated alkyl, alkenyl, alkynyl, and alkoxyl.

MAMS 10 may be conceptualized as comprising secondary building units (SBU), the metal clusters forming one SBU 12 (e.g., Ni₈(μ₃-OH)₄) (FIGS. 3 a and 3 b) and the ligands forming a second SBU (e.g., 5-tert-butyl-1,3-benzenedicarboxylate (BBDC)). In this MAMS, denoted MAMS-1, the eight octa-nickel atoms are divided into four pairs by a two-fold axis through the center of the cluster, a first Ni pair is bound to five carboxylate O atoms from four 5-BBDC ligands and one μ₃-OH groups, a second Ni pair is bound to three carboxylate O atoms and three μ₃-OH groups, a third Ni pair is bound to four carboxylate O atoms and one μ₃-OH group, and a fourth Ni pair is bound to two carboxylate O atoms and one μ₃-OH group. Thus, in MAMS-1, every octa-nickel cluster connects 12 BBDC ligands, and every BBDC ligand binds two octa-nickel cluster to comprise a tri-layer, with a hydrophilic cluster layer sandwiched by two hydrophobic GGDC layers (FIG. 1 a). There are one-dimensional pores 16 along the α axis (FIG. 1 a) in the middle layer, in which guest water solvates reside. (FIG. 2 a.) The tri-layers, with their exposed hydrophobic exteriors 14, pack along the c axis through van der Waals forces (FIG. 1 a), thus forming hydrophobic pores 18 between adjacent tri-layers 22.

The metal cluster SBUs 12 are generally of the form M_(X)A_(Y) wherein M is preferably a transition metal, Al, Ge, or Ga and A comprises oxo, hydroxyl, sulfur, and halogen. The ligand SBUs are shown generally above and in FIGS. 8 a-8 e. Other exemplary metal cluster SBUs include Cu₂, Co₂, Mn₂, and Zn₂. Other exemplary MAMS include Cu₂(MBPDC)₂, Co₂(BBPDC)₂, Mn₂(BBPDC)₂, Zn₂(BBPDC)₂, and Cu₂(BBPDC)₂, wherein BBDC is 5-tert-butyl-1,3-benzenedicarboxylate, MBPDC is 4′-methyl-biphenyl-3,5-dicarboxylate, and BBPDC is 4′-tert-butyl-biphenyl-3,5-dicarboxylate.

Freshly synthesized and isolated MAMSs 10, however, are not active for adsorption. (FIGS. 2 a-2 c, 3 a, 3 b, and 12-14.) As shown for MAMS-1 (10, FIGS. 1 a and 1 b) in FIGS. 12 and 13, for example, H₂ and CO are not adsorbed until MAMS-1 10 is activated at an elevated temperature. This phenomenon is further illustrated generically in FIGS. 2 a and 2 b. A thermogravimetric analysis of MAMS-1 10, for example, revealed a loss of eight guest water molecules from 50 deg. C. to 120 deg. C. and the release of eight bound aqua ligands per formula unit when heated to 250 deg. C. (FIG. 10.) For MAMS-1, the pre-activation formula is: Ni₈(μ₃-OH)₄(C₁₂H₁₂O₄)₆(H₂O)₈.8H₂O and the activated formula is: Ni₈(μ₃-OH)₄(C₁₂H₁₂O₄)₆, where C₁₂H₁₂O₄ is 5-tert-butyl-1,3-benzenedicarboxylate (BBDC).

The hydrophilic pores 16 alone appear to not be responsible for the gas uptake. In fact, they seem to account for a very minor part of the adsorption. As shown for MAMS-1 10, for example, in FIG. 9 a, at 77 deg. K, only H₂ can enter the hydrophobic pores 18, showing significant uptake. At 77 deg. K, other molecules (CO, O₂, and N₂) cannot move beyond the hydrophilic pores 16 and the uptake of these gases is very low.

The hydrophilic pores 16, preferably exhibiting an atom-to-atom distance of about 8 Angstroms (Å) and an atom-to-atom distance of about 5 Å considering van der Waals radii, reach the structural limits the MAMS 10 and are in communication with the hydrophobic pores 18 via the molecular gates 26 and gateways 27 (FIGS. 1 a, 2 a-2 c, 4 a, and 4 b). Again, while not wishing to be bound by any particular theory, it is believed molecular gates 26, formed by a pair of ligand groups (FIGS. 8 a-8 e) which are held in close proximity via van der Waals interaction, which is readily weakened by thermal vibration, preferably exhibit a continuously thermally-adjustable size in the single-digit Angstrom range, preferably between about 2.9 Å and about 5 Å. As will be appreciated by those skilled in the art, it is likely that the gate effect is not just the result of thermal vibration of the hydrophobic moiety alone as the other portions of the molecular structure, particularly the phenyl ring, for example, will also contribute somewhat. When the temperature is precisely controlled, any gate/gateway 26, 27, effect or mesh, size within this range can be accurately attained. This molecular gate/gateway 26, 27 effect size range covers almost all commercially-important gas separations. For example, H₂/N₂, H₂/CO, N₂/O₂, N₂/CH₄, CH₄/C₂H₄ and C₂H₄/C₃H₆, wherein the molecules are separated from their respective pair. Exemplary kinetic diameters include H₂ (2.89 Å), O₂ (3.46 Å), N₂ (3.64 Å), CO (3.76 Å), CH₄ (3.8 Å), C₂H₄, (3.8 Å), C₃H₆, (4.5 Å), and SF₆ (5.5 Å).

In operation, it is believed gas molecules enter the hydrophobic pores 18 through the hydrophilic pores 16 and gates 26 in the hydrophobic/hydrophilic interface. (FIGS. 1 a, 1 b, and 2 a-2 c.) It has been found, for example, that when the kinetic diameter of the gas molecule (e.g., SF₆ at 5.5 Å) exceeds the size of the hydrophilic pores 16 (5.0 Å considering van der Waals radii), no meaningful uptake was observed in an adsorption study. (FIGS. 2 a-2 c and 16.) The upper limit of the hydrophilic pore 16 is also consistent with an adsorption study on iso-C₄H₁₀ (5.0 Å) which entered MAMS-1 10 at room temperature (295 deg. K). (FIG. 16.)

The temperature-dependent molecular-gating effect does not appear to arise from simple thermal expansion of the MAMS 10 framework. As shown in FIG. 21, for example, the lattice size parameters for Ni₈(μ₃-OH)₄(BBDC)₆ (MAMS-1) 10 show virtually no response to temperature over a wide range. The opening of the gate 26 appears to be controlled by the amplitude of thermal vibration. FIG. 20, for example, shows temperature versus size of the molecule allowed to pass through the gate opening for MAMS-1 10. The data point in FIG. 20 at 77 deg. K gives an under-estimation of the gate opening because a gas molecule with a size between 2.89 Å (H₂) and 3.40 Å (Ar) is not available for gas adsorption studies. Ignoring the point at 77 deg. K, the gate opening D in Angstroms, and temperature T deg. K, and be related by a linear equation

D=0.0076T+2.76,

with a correlation coefficient of 0.996. This equation can be used to predict if a gas molecule will be able to enter the gate 26 at a certain temperature. It may also be used to find the best temperature for the separation of a mixture. More generally, the linear relationship between mesh size and temperature can be represented as

D=D ₀ +αT,

where D is the mesh size in Angstroms at temperature T (deg. K), D₀ is the mesh size in Angstroms at 0 deg. K, and α is a constant.

In synthesizing a MOF-based MAMS 10, a source of metal ions and a source of amphiphilic ligands (FIGS. 8 a-8 e), each ligand comprising a hydrophobic moiety 14 and a functionalized hydrophilic moiety (FIGS. 8 a-8 e), are mixed together in a first suitable solvent. The mixture is then heated to a first temperature, optionally at a first heating rate, held at the first temperature for a suitable length of time, and cooled to a second temperature, whereby MAMS are synthesized. Suitable solvents include, but are not limited to, H₂O/ethylene glycol, dimethyl sulfoxide (DMSO), dimethylacetamide (DMA), and dimethylformamide (DMF). First temperatures range, for example, between 75 deg. C. to 210 deg. C. Suitable hold temperatures range, for example, between 18 hours and 24 hours. After washing, the fresh MAMS are activated by suitable heating and pressure to remove guest solvent molecules and bound solvent molecules.

Example

The MAMS illustrated in FIG. 1 a (MAMS-1) was synthesized by a solvothermal reaction between Ni(NO₃)₂ and 5-tert-butyl-1,3-benzenedicarboxylate (BBDC). 5-tert-butyl-1,3-benzenedicarboxylic acid (H₂BBDC), (0.075 g, 0.34 mmol) and Ni(NO₃)₂.6H₂O (0.15 g, 0.51 mmol) in 7.5 ml H₂O/ethylene glycol (volume ratio 4:1) were placed in a 20 ml Teflon® (E. I. du Pont de Nemours and Company, Wilmington, Del.) container and sealed in an autoclave. The autoclave was heated to 210 deg. C. (heating rate 2 deg. C./min) in a programmable oven at which it stayed for 24 hours before being cooled to room temperature (cooling rate 0.5 deg. C./min). The light green needle-like crystals obtained were washed with distilled water and methanol to give pure MAMS-1 with the formula Ni₈(μ₃-OH)₄(C₁₂H₁₂O₄)₆(H₂O)₈.8H₂O (55 percent yield based on H₂BBDC). Elemental analysis, calculated (percent): C, 40.28; H, 5.07; O, 32.79. found: C, 40.69; H, 5.07; O, 33.05. IR (cm⁻¹): 3305 (w, br), 2960 (m), 1033 (s), 865 (s), 785 (m).

Single crystal X-ray determination (FIG. 11) was performed on a Bruker Smart Apex® diffractometer (Bruker AXS, Inc., Madison, Wis.) using Mo—Kα radiation (λ=0.71073 Å). The data were collected on a crystal with dimensions of 0.23 mm×0.08 mm at −60 deg. C. A total of 1321 frames of data were collected using ω-scans with an increment of 0.3 deg. and a counting time of 60 sec/frame. The raw data were processed using SAINT+® (Bruker) to yield the HKL file. Adsorption corrections were applied using SADABS® (Bruker). Direct methods were used to solve the structure, which was refined by full-matrix least-squares on F² with anisotropic displacement parameters. The hydrogen atoms on carbon and oxygen atoms were calculated in ideal positions with isotropic displacement parameters set to 1.2*U_(eq) of the attached atom.

Crystal Data - MAMS-1 Empirical formula C₃₆H₅₂Ni₄O₂₁ Formula weight 1055.62 Temperature 213 (2) K Wavelength 0.71073 Å Crystal system, space group Monoclinic, P2 (1)/c Unit cell dimensions a = 10.9685 (2) Å alpha = 90.00°; b = 11.308 (2) Å beta = 96.781(3)°; c = 38.405 (7) Å gamma = 90.00°. Volume 4730.1 (14) Å³ Z, Calculated density 4, 1.482 g/cm³ Absorption coefficient 1.641 mm⁻¹ F (000) 2192 Crystal size 0.23 × 0.08 × 0.08 mm Theta range for data collection 1.87 to 23.33° Reflections collected/unique 16860/6818 [R(int) = 0.0668] Completeness to theta = 18.91 99.2% Absorption correction Empirical Max. and min. transmission 1.000 and 0.648 Refinement method Full-matrix least-squares on F² Data/restraints/parameters 6818/18/581 Goodness-of-fit on F{circumflex over ( )}2 1.021 Final R indices [I > 2sigma(I)] R₁ = 0.0609, wR₂ = 0.1456 R indices (all data) R₁ = 0.0928, wR₂ = 0.1624

Thermogravimetric analysis (TGA) of MAMS-1 (9.8 mg) was performed with a Perkin-Elmer TGA 7 Thermogravimetric Analyzer under 50.0 ml/min flow of N₂. (FIG. 10.) The first weight loss of 6.72 percent (calculated: 6.71 percent) from 50 deg. C. to 120 deg. C. corresponds to the loss of eight free H₂O molecules, followed by the weight loss of 6.42 percent (calculated: 6.71 percent) corresponding to eight coordinated H₂O molecules from 120 deg. C. to 250 deg. C. Beyond 400 deg/C., the framework decomposes completely.

Gas adsorption measurements were measured with an SA 3100 surface area and pore size analyzer (Beckman Coulter, Inc., Fullerton, Calif.). The sample was held under dynamic vacuum (<10⁻³ Torr) at 200 deg. C. overnight to remove the free and coordinated water molecules. Before the measurement, the sample was evacuated again by using the “outgas” function of the surface area analyzer for one hour at 200 deg. C. A sample of 40.0 mg was used for N₂ (99.999 percent) adsorption measurement, and was maintained at 77 K with liquid nitrogen. (FIG. 9 a.) In the H₂ adsorption measurement, high purity hydrogen (99.9995 percent) and a 40.0 mg sample were used. The regulator and pipe were flushed with hydrogen before connecting to the analyzer. The internal lines of the instrument were flushed three times by utilizing the “flushing lines” function of the program to ensure the purity of hydrogen. The measurement was maintained at 77 deg. K with liquid nitrogen. (FIGS. 12 and 17.) Similar to the procedures used for H₂ measurement at 77 K, highly pure O₂ (99.99 percent) (FIG. 9 a), CO (99.99 percent) (FIG. 9 a), CH₄ (99.997 percent) (FIG. 18), C₂H₄ (99.5 percent) (FIGS. 15 (175 deg. K) and 19 (195 deg. K)), C₃H₆ (99.5%) (FIGS. 9 c and 9 f), iso-C₄H₁₀ (99.5 percent) (FIG. 16), SF₆ (99.8 percent) (FIG. 16) and CO₂ (99.99 percent) (FIG. 13) were used for their respective gas adsorption measurements. All the gases used for the measurements were purchased from Linde Gas LLC, Cincinnati, Ohio. To prevent condensation of CO and O₂ at 77 K, the pressure ranges were below 448 Torr and 156 Torr, respectively. To prevent condensation of O₂ at 87 K, the pressure range was below 466 Torr. To prevent condensation of C₂H₄ at 143 K, the pressure range was below 120 Torr. To prevent condensation of C₃H₆ at 195 K, the pressure range was below 110 Torr. To prevent condensation of iso-C₄H₁₀ at 241 K, the pressure range was below 210 Torr. For all adsorption isotherms, P₀ represents a relative saturation pressure given by the SA 3100 during the measurements. At 77 deg. K, P₀ was 757 Torr for H₂ (FIG. 12) and N₂, 441 Torr for CO, and 151 Torr for O₂. At deg. 87 K, P₀ was 757 Torr for CO and N₂ and 465 Torr for O₂. At 113 deg. K, P₀ was 757 Torr for CO, CH₄, and N₂. At 143 K, P₀ was 757 Torr for CH₄ and 118 Torr for C₂H₄. At 175 deg. K, P₀ was 757 Torr for C₂H₄. At 195 deg. K, P₀ was 757 Torr for C₂H₄ and CO₂ and 108 Torr for C₃H₆. At 241 deg. K, P₀ was 757 Torr for C₃H₆ and 205 Torr for iso-C₄H₁₀. At 298 K, P₀ was 757 Torr for iso-C₄H₁₀ and SF₆.

Referring now to FIG. 9 a, at 77 deg. K, MAMS-1 excludes CO, N₂, and O₂, but allows H₂ to be adsorbed. When the temperature is raised to the temperature of liquid argon (87 deg. K), FIG. 9 b shows that only a small amount of CO or N₂ is adsorbed by MAMS-1. At that temperature, however, MAMS-1 can take up a significant amount of O₂. The adsorption isotherm of O₂ shows typical Type-I behavior. Dioxygen (3.46 Å) can be selectively adsorbed from a mixture with N₂ (3.64 Å) and CO (3.76 Å), which would imply that at 87 deg. K, the gate 26 opens to about 3.5 Å.

Referring now to FIG. 9 c, at 113 deg. K, MAMS-1 can take up a moderate amount of N₂, but relatively low quantities of CO (3.76 Å) and CH₄ (3.8 Å). Thus, the gate 26 opens wide enough to enable N₂ (3.64 Å) to be adsorbed, but molecules with larger kinetic diameters such as CO and CH₄ are not.

Referring now to FIG. 9 d, MAMS-1 appears to distinguish between CH₄ and C₂H₄ (3.8 Å) at 143 deg. K. In addition, C₂H₄ is distinguished from C₃H₆ at 195 deg. K (FIG. 9 e) and C₃H₆ (4.5 Å) from iso-C₄H₁₀ (5.0 Å) at 241 deg. K (FIG. 9 f).

The 4′-methyl-biphenyl-3,5-dicarboxylic acid (H₂MBPDC) precursor for the ligand 4′-methyl-biphenyl-3,5-dicarboxylate (MBPDC) (FIG. 8 a) is synthesized as shown in FIG. 6.

The 4′-tert-butyl-biphenyl-3,5-dicarboxylic acid (H₂BBPDC) precursor for the ligand 4′-tert-butyl-biphenyl-3,5-dicarboxylate (BBPDC) (FIG. 8 a) is synthesized as shown in FIG. 7.

This detailed description in connection with the drawings is intended principally as a description of the presently preferred embodiments of the invention, and is not intended to represent the only form in which the present invention may be synthesized, formed, or utilized. The description sets forth the designs, functions, means, and methods of implementing the invention in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions and features may be accomplished by different embodiments that are also intended to be encompassed within the spirit and scope of the invention and that various modifications may be adopted without departing from the invention or scope of the following claims. 

1. A method of synthesizing a metal-organic framework-based mesh-adjustable molecular sieve (MAMS), comprising the steps of: dissolving a source of metal ions and a source of amphiphilic ligands to form a solution, each ligand comprising a hydrophobic moiety and a functionalized hydrophilic moiety; and crystallizing the solution under predetermined conditions to form a predetermined MAMS.
 2. The method of claim 1, wherein the solution is heated to a predetermined temperature.
 3. The method of claim 1, wherein the solution is cooled to a predetermined temperature.
 4. The method of claim 1, further comprising the step of washing the MAMS with a suitable solvent.
 5. The method of claim 4, further comprising the step of heating the MAMS to a predetermined temperature, whereby the MAMS become activated for gas adsorption.
 6. The method of claim 1, wherein the source of metal ions comprises at least one ion of metals selected from the group consisting of transition metals.
 7. The method of claim 6, wherein the transition metals are selected from the group consisting of period four transition metals.
 8. The method of claim 7, wherein the period four metals are selected from the group consisting of Cr, Ni, Cu, Co, Mn, and Zn.
 9. The method of claim 1, wherein the source of metal ions comprises at least one ion of metals selected from the group consisting of Al, Ge, and Ga.
 10. The method of claim 1, wherein the amphiphilic ligand is selected from the group consisting of:

wherein: X is at least one of carboxylate, cyano, phosphonate, sulfonate, imidazolate, pyridine, pyrazolate, and tetrazolate; and Y is at least one of tert-butyl, methyl, isopropyl, trifluoromethyl, butoxyl, and butylsulfonyl, alkyl, halogenated alkyl, alkenyl, alkynyl, and alkoxyl.
 11. The method of claim 1, wherein the metal ions and the amphiphilic ligands are dissolved in an aqueous solvent.
 12. The method of claim 11, wherein the aqueous solvent comprises ethylene glycol.
 13. A method of separating gaseous molecules, comprising contacting the MAMS synthesized by the method of claim 1 with a mixture of molecules selected from the group consisting of H₂/N₂, H₂/CO, N₂/O₂, N₂/CH₄, CH₄/C₂H₄ and C₂H₄/C₃H₆, wherein the molecules are separated from their respective pair.
 14. A method of adsorbing a molecular species selected from the group consisting of H₂, N₂, O₂, CO, Ar, CH₄, C₂H₄, and C₃H₆, comprising contacting the MAMS by the method of claim 1 with a mixture of molecular species comprising one of H₂, N₂, O₂, CO, Ar, CH₄, C₂H₄, and C₃H₆. 